The present invention generally relates to a data transmission system and more particularly relates to a system of transmitting data, which is supplied by a rotational speed sensor in the form of an alternating signal, as well as additional data via a joint signal line.
Rotational speed sensors are required, among others, in motor-vehicle engineering in order to produce one of the most important input quantities of many vehicle control systems. For example, anti-lock systems (ABS), traction slip control systems (TCS) and driving stability control systems (DSC, ASMS) known in the art are based on the constant measuring and evaluating of the rotational behavior of the individual vehicle wheels. The rotational speed data of each individual wheel are sensed and transmitted to an evaluating electronics by way of one or more wires (i.e. cable).
Rotational speed sensors of most different types and output capacities are provided. The sensors employed in automotive vehicles are mainly comprised of an encoder configured as a toothed disc, hole disc, etc., and rotating with the wheel, and a stationary measuring data transducer. For technological and cost reasons, inductive sensors or transducers have previously been preferred, wherein the encoder produces an alternating electrical signal representative of the rotation of the wheel. The frequency of the alternating signal is evaluated in a different fashion for the acquisition of information on rotational speeds.
Active rotational speed sensors are gaining in significance. An active sensor of this type has already been described in WO 95/17680 (P 7805). The stationary-part of the sensor in such active sensors includes, for example, a magnetoresistive sensor element with a permanent magnet used as a biassing magnet, and also electronic circuits. An active sensor requires current supply. The output signal of the active sensor is a binary current signal which is composed of load-independent currents of different amplitudes. The rotational speed information is included in the frequency or in the change between the two current levels. Prior art sensors of this type produce a square-wave signal whose frequency represents the measured rotational speed.
The manufacturing costs involved in automotive vehicle control systems of the type described herein are of major importance under marketing aspects. There is relatively much effort and structure needed for sensing the rotational behavior of all wheels, and transmitting and evaluating the information. In addition, further pieces of information of most different types occurring on each wheel, such as brake lining wear, the air slot between encoder and sensor, the temperature of the brakes or brake fluid temperature, the brake fluid condition, etc., must be established and transmitted to the evaluating circuit.
An object of the present invention is to reduce the total expenditure incurred for measuring and evaluating the information sensed on the wheel and the complexity of transmitting the measured data from the wheels to the evaluating circuit (expenditure in wiring), or, in other words, to improve the utilization of the wiring arrangement between the individual (active) sensors and the central evaluating circuit.
It has been found that this object can be achieved by the method described of the present invention wherein a sequence of pulses of a given duration is produced from the alternating signal, and the pulse intervals or interpulse periods contain information on rotational speed, and that the additional data are transmitted in the interpulse periods, and the transmission of the additional data is triggered or synchronized by the individual pulses.
Thus, the present invention is based on the recognition that the cable (single-wire or two-wire cable) between the individual wheel sensors and the central evaluating electronics can additionally be used for the transmission of further wheel data, for example, about the brake lining wear, the air slot between the encoder and the rotational speed sensor element, about the temperature on the wheel, the compression travel of the vibration dampers, and/or about many other data, and that this is possible without additional effort and structure by using the method of the present invention.
In an embodiment of the method of the present invention, the additional data is included in a sequence of binary signals or in a bit sequence which is transmitted following each sensor pulse or synchronization pulse. Appropriately, the total duration of the signal sequence or bit sequence is shorter than the interpulse period which occurs between the successive sensor pulses at maximum rotational speed.
Further, it has been proven favorable that the time basis for the sensor pulses and for the binary signals which contain the additional data is produced by way of a common oscillator or clock pulse generator. The accuracy of frequency of such an oscillator must satisfy only low requirements.
Still further, it is favorable to use an active sensor for determining the rotational speed, the sensor supplying the rotational speed data in the form of load-independent currents of different amplitudes, and to also transmit the additional data in the form of currents of a predetermined amplitude. In this case, a basic current value which is sufficient for the operation of the active sensor, a mean current value for transmitting the additional data and a top current value for representing the sensor pulses are predetermined on the transmission line. Besides, the information when a defined minimum current value falls below and when a maximum current value is exceeded on the transmission line can be used for error detection, such as line interruption, shortcircuit to ground or battery, etc.
Another improvement of the method of the present invention is achieved when, upon wheel standstill recognition or in the absence of a sensor pulse beyond a predetermined time period, a secondary or auxiliary synchronization pulse is generated which triggers the transmission of the additional data. This is because there are wheel data, such as brake lining wear indicator and many others, which should absolutely be transmitted and evaluated even at standstill of the vehicle.
However, the transmission of the wheel sensor data has priority. Therefore, according to the present invention, the data flow is interrupted and restarted by a sensor pulse as soon as the sensor pulse occurs during a transmission of additional data, triggered by the secondary synchronization pulse.